Using taghelpers in razor views using a model type which is loaded runtime

[DavidEmanuelsen]

Hi,

I have created an application which uses a custom fileprovider for loading razor views from disk / database, and these views uses models coming from an assembly which is loaded runtime. I've used the RazorViewEngineOptions.CompilationCallback to load this assembly and the compilation of the views works - except for when using asp-for etc. (The compilation callback was implemented as per the example given in #4423)

The model object is created using Activator.CreateInstance where the type is coming from the external assembly.

The errors I'm getting when using taghelpers are:

• Type expected
• Cannot convert lambda expression to intended delegate type because some of the return types in the block are not implicitly convertible to the delegate return type

These errors don't contain much detail, but I'm assuming the problem is related to the model type which is loaded runtime. Any suggestions on how I can get this to work is greatly appreciated.

[dougbu]

If any of the above errors are actually thrown exceptions, please provide the stack traces.

[Eilon]

@DavidEmanuelsen indeed, where are you seeing these errors, and can you show us any relevant code from your app?

[DavidEmanuelsen]

I have project containing the model to be used which implements an interface which is also know by the running application. This project / class library is build seperately, and in the main application I create an instance of the model like this:

Assembly externalAssembly = AssemblyLoadContext.Default.LoadFromAssemblyPath(assemblyPath);
var type = Type.GetType(serviceNamespace 
            + "." + serviceImplementationName 
            + "," + serviceNamespace);

return (IServiceModel)Activator.CreateInstance(type);

The compilation callback looks like this:

services.Configure<RazorViewEngineOptions>(options =>
{
        var previous = options.CompilationCallback;
        options.CompilationCallback = (context) =>
        {
               previous?.Invoke(context);
               MetadataReference[] references = serviceAssemblyReferences.ToArray();

               context.Compilation = context.Compilation.AddReferences(references);
         };

         options.FileProviders.Add(new CustomFileProvider());
});

Where the serviceAssemblyReferences-list is populated with new items as needed before returning from the controller (creating new items using MetadataReference.CreateFromFile(path))

I'm having some issues debugging the application at the moment (not able to run it directly from VS, and when I use dotnet publish I can't attach to the running process), but I will update with stacktraces as soon as I can get them. Currently I'm only getting what the developer exception page is showing me when using <input asp-for="FirstName" type="text" /> or @Html.EditorFor(m => m.FirstName)

[DavidEmanuelsen]

Just realized I can give the type using @Html.EditorFor<MyModelType, int>(...).. How can the same be done for the taghelpers?

[Eilon]

@DavidEmanuelsen - any chance you can replicate this scenario without the custom assembly loads, file system, etc.? In other words, can you create a regular CSHTML file on disk that uses regular models in regular CS files? I'm guessing you'll hit the exact same error, but it might be easier to replicate and debug from that.

[dougbu]

Just realized I can give the type using @Html.EditorFor<MyModelType, int>(...). How can the same be done for the tag helpers?

What is the advantage you see in calling EditorFor() with explicit type parameters? Suggest focusing on why can't the compiler infer those parameters from the expression.

In any case you'll need parentheses around the generic method call: @(Html.EditorFor<MyModelType, int>(...)).

Tag helpers have no equivalent to this. While it's possible to explicitly create a ModelExpression containing the correct information e.g. CreateModelExpression(m => m.Name) that instance can't be assigned to an asp-for attribute value -- that value is always part of a lambda expression in the generated code.

[DavidEmanuelsen]

I've tried reproducing it without any assembly loads etc, but no luck. I'm only able to reproduce it when loading the assembly containing the model runtime. I've created a simple project which illustrates my issue.

Controller action:

public ActionResult Index()
{
     return View(Activator.CreateInstance(Type.GetType("ModelProject.MyModel,ModelProject")));
}

View (now contained within the running application instead of loading from a custom location):

@model ModelProject.MyModel
@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers

<input asp-for="MyProperty" type="text" />

Startup:

    public class Startup
    {
        public static List<MetadataReference> serviceAssemblyReferences = new List<MetadataReference>();

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddMvc();
            services.Configure<RazorViewEngineOptions>(options =>
            {
                var previous = options.CompilationCallback;
                options.CompilationCallback = (context) =>
                {
                    previous?.Invoke(context);

                    context.Compilation = context.Compilation.AddReferences(serviceAssemblyReferences.ToArray());
                };
            });
        }

        public void Configure(IApplicationBuilder app)
        {
            app.UseDeveloperExceptionPage();
            app.UseStaticFiles();
            app.UseMvcWithDefaultRoute();
        }

        public static void Main(string[] args)
        {
            var ctx = new MyAssemblyLoadContext();
            AssemblyLoadContext.InitializeDefaultContext(ctx);

            var host = new WebHostBuilder()
                .UseDefaultHostingConfiguration(args)
                .UseIISIntegration()
                .UseKestrel()
                .UseStartup<Startup>()
                .Build();

            host.Run();
        }
    }

    public class MyAssemblyLoadContext : AssemblyLoadContext
    {
        protected override Assembly Load(AssemblyName assemblyName)
        {
            string path = "C:\\ServiceAssemblies\\" + assemblyName.Name + ".dll";
            Startup.serviceAssemblyReferences.Add(MetadataReference.CreateFromFile(path));

            return LoadFromAssemblyPath(path);
        }
    }

The ModelProject-assembly is build seperatly, but it only contains one class as follows:

public class MyModel
{
    public int MyProperty { get; set; }

     public MyModel()
     {
         MyProperty = 1337;
     }
}

[Eilon]

@dougbu can you try to repro and see what this error is about?

[dougbu]

@Eilon 🆗

[dougbu]

I can reproduce this compilation error without the custom AssemblyLoadContext.

The CSharpCompilation change is at least partially doing the Right Thing^™️. Without the added MetadataReference, Roslyn errors include The type or namespace name 'ModelProject' could not be found (are you missing a using directive or an assembly reference?).

Hard to tell exactly what's going wrong due to the messed-up line numbers in the errors. But based on the columns and the general class of compilation failures, problems seem focused on ModelExpressionProvider.CreateModelExpression(ViewData, __model => __model.MyProperty); in the generated code. ViewData is of type ViewDataDictionary<ModelProject.MyModel> and the lambda is of type Expression<Func<ModelProject.MyModel, int>>. The compiler must be having problems inferring these types.

Will continue looking.

/cc @davidfowl any suggestions on where to go next?

[sebastienros]

One of Orchard's contributor had to move the compilation callback call to fix a related issue regarding compiling Views with references to dynamically loaded assemblies:

https://files.gitter.im/OrchardCMS/Orchard2/6xVS/roslyn.png